1. Field of the Invention
The present invention relates to a lock detector of an automatic gain control loop, and more particularly, to a lock detector that determines a convergence degree of an automatic gain control loop of a digital broadcast receiver to thereby control a gain thereof.
2. Discussion of the Related Art
A digital television (TV) has been recently developed for reproducing an impression of a theater in the inner room. Compared to the existing analog TV, the digital TV has very high screen resolution (for example, 1080×1920) and a wide screen, and provides a sound of CD quality over multi-channels (for example, 5.1 channels).
Many countries, such as USA, Japan and European countries have respectively prepared a broadcast standard for the digital TV and are promoting standardization for the digital TV. Particularly, USA has adopted a vestigial sideband (VSB) technique (proposed by Zenith Corporation) as a transmission format, an MPEG technique as a video compression format, Dolby AC-3 as an audio compression format, and a display format compatible with the existing display method.
In the digital broadcast system, a transmission signal of a transmitter is constant in its gain. However, the transmission signal is varied in its gain while transmitted through various channel to a receiver. In this manner, signals of various gains are received at the receiver. However, most digital parts of a receiver are designed on the assumption that a signal of a constant gain is inputted to the receiver. Accordingly, an analog signal inputted to a receiver should be adjusted in its gain to have a constant gain, prior to conversion into a digital signal.
An automatic gain controller (AGC) device performs such a gain adjustment operation. The AGC device determines a gain of a current input signal from an average or power of an input signal. At this time, the AGC controls amplifiers in analog circuits of a radio frequency (RF) stage and an intermediate frequency (IF) stage to thereby cause the input signal to have a desired size.
FIG. 1 is a block diagram of a general VSB receiver having an AGC.
A VSB receiver shown in FIG. 1 is an example of a receiver that uses an automatic gain controller (AGC) for directly controlling gains of a radio frequency (RF) and an intermediate frequency (IF).
Referring to FIG. 1, the VSB receiver is constructed to include a tuner 1, an IF AGC amplifier 2 and a VSB demodulator (VSB Rx chip) 3. The tuner 1 tunes only a channel frequency selected by a user from RF signals received through an antenna, and then controls the tuned RF signal to thereby convert the tuned RF signal into an IF signal. The IF AGC amplifier 2 controls and amplifies a gain of the IF signal. The VSB demodulator 3 VSB-demodulates the gain-controlled IF signal, and outputs gain control signals (RF AGC control, IF AGC control) respectively to the tuner 1 and the IF AGC amplifier 2, and directly controls gains of the RF and IF signals. Here, the VSB demodulator 3 is equipped with an AGC 3-1 for generating an IF gain control signal (IF AGC control) and an RF gain control signal (RF AGC control).
An operation of the VSB receiver will now be described with reference to FIG. 1.
Firstly, when a VSB-modulated RF signal is received through the antenna, the tuner 1 selects a desired channel frequency by tuning and then converts a tuned RF signal into an IF signal to thereby output the IF signal to the IF AGC amplifier 2. The IF AGC amplifier controls a gain of the IF signal to thereby output the gain-controlled IF signal to the VSG demodulator 3. The VSB demodulator 3 VSB-demodulates the gain-controlled IF signal.
At this time, in order to maintain a constant signal gain, the AGC 3-1 of the VSB demodulator 3 compares the strength of the VSB-demodulated signal with a pre-stored gain information value. If a gain of the received signal is determined to be small from the result of the comparison, the VSB demodulator 3 generates a gain-up signal for increasing the received signal's gain. Otherwise, if a gain of the received signal is determined to be large from the result of the comparison, the VSB demodulator 3 generates a gain-down signal for decreasing the received signal's gain. The VSB demodulator 3 then outputs IF gain control signals (the gain-up signal and the gain-down signal) to the IF AGC amplifier 2.
The IF AGC amplifier 2 increases or decreases a gain of the IF signal according to the gain-up signal or the gain-down signal to thereby cause an IF signal to have a desired gain.
In an actual gain control method, after a gain is set to a maximum gain that the VSB receiver can have at the power-on time or at the time of the channel change, the tuner 1 controls a gain of the RF signal through the RF gain control signal in advance to the strength of an input signal.
If the strength of an output signal of the VSB demodulator 3 is not a desired strength even though a gain control capability of the tuner 1 reach the limit, the VSB demodulator 3 controls a gain of the IF AGC amplifier 2 through the IF gain control signal.
In this manner, the AGC 3-1 is operated by a feedback loop adequate for adjusting a gain of a signal received through an antenna.
Accordingly, it is important to adjust a convergence characteristic of the AGC 3-1 that is operated by a feedback loop.
That is, although a loop gain (or an operational area) should be increased for a rapid convergence characteristic, a signal level becomes unstable or an MSE (mean square error) of a symbol recovered by a receiver becomes great when the loop gain is increased too high.
Accordingly, it is important to develop a receiver that not only has a rapid convergence characteristic but also controls a gain stably.